materials Review A Review of Bioactive Glass/Natural Polymer Composites: State of the Art Rachele Sergi, Devis Bellucci and Valeria Cannillo * Dipartimento di Ingegneria Enzo Ferrari, Università degli Studi di Modena e Reggio Emilia, Via P. Vivarelli 10, 41125 Modena, Italy; [email protected] (R.S.); [email protected] (D.B.) * Correspondence: [email protected]; Tel.: +39-059-205-6240 Received: 13 November 2020; Accepted: 4 December 2020; Published: 6 December 2020 Abstract: Collagen, gelatin, silk fibroin, hyaluronic acid, chitosan, alginate, and cellulose are biocompatible and non-cytotoxic, being attractive natural polymers for medical devices for both soft and hard tissues. However, such natural polymers have low bioactivity and poor mechanical properties, which limit their applications. To tackle these drawbacks, collagen, gelatin, silk fibroin, hyaluronic acid, chitosan, alginate, and cellulose can be combined with bioactive glass (BG) nanoparticles and microparticles to produce composites. The incorporation of BGs improves the mechanical properties of the final system as well as its bioactivity and regenerative potential. Indeed, several studies have demonstrated that polymer/BG composites may improve angiogenesis, neo-vascularization, cells adhesion, and proliferation. This review presents the state of the art and future perspectives of collagen, gelatin, silk fibroin, hyaluronic acid, chitosan, alginate, and cellulose matrices combined with BG particles to develop composites such as scaffolds, injectable fillers, membranes, hydrogels, and coatings. Emphasis is devoted to the biological potentialities of these hybrid systems, which look rather promising toward a wide spectrum of applications. Keywords: natural polymers; bioactive glasses; composites; mechanical properties; biological performance 1. Introduction Over the past 40 years, life expectancy in industrialized countries has continued to rise thanks to many factors such as healthier nutrition, health care system and public health efforts, medical treatments, and more salubrious lifestyles [1,2]. However, this progress goes hand in hand with an increase of pathologies and work-related accidents such as spinal problems, arthritis, joint dysfunction, traumatic injuries, and lacerations [3,4]. Such diseases imply dramatic suffering for patients and might cause severe and long-term pain, work limitation, and disability. In this context, biomaterials’ science can make an important contribution to medicine, thanks to the possibility to design increasingly advanced prostheses and implants to be used in several clinical applications to correct and improve irregularities and abnormalities (i.e., spinal rods, pacemaker, stent), to assist in recovery from injury (structural, pharmaceutical effects), and to replace body parts that lose function (total hip, heart). Therefore, new biomaterials with greater biological response, biocompatibility, and thermal and mechanical properties have been increasingly studied [5,6]. Biomaterials, which should be biocompatible, intrinsically non-toxic, non-carcinogenic, and non-allergenic, can be classified as first generation, second generation, and third generation according to their response in the host. First-generation biomaterials had the aim to achieve a proper combination of mechanical and biological properties that correspond to those of the replaced tissue. Typically, these biomaterials do not cause or undergo chemical or biological changes in the host. On the contrary, second-generation biomaterials can promote a specific biological response on the biomaterials’ surface. Materials 2020, 13, 5560; doi:10.3390/ma13235560 www.mdpi.com/journal/materials Materials 2020, 13, 5560 2 of 38 Therefore, second-generation biomaterials can stimulate the formation of new bone by inducing a controlled action and reaction mechanisms in a biological environment [5,7]. On the other hand, a specific response at the molecular level is promoted by third-generation biomaterials [8]. In fact, such devices can be gradually degraded and substituted by living host tissues. Biomaterials may include ceramics, glasses, metals, and polymers with their specific properties [9,10]. Since polymers are the most versatile class of biomaterials, they have been extensively used in industrial applications such as farming, food sectors, pharmaceutical, and biomedical fields. Polymers are classified into synthetic and natural types: a large number of synthetic polymers are synthesized from petroleum oil through a series of chemical reactions, while natural polymers are extracted from animal waste or plants in nature [11–13]. Synthetic polymers can be classified as (i) hydrophobic, non-water absorbing materials (i.e., polyethylene (PE), polypropylene (PP), poly(methylmethacrylate) (PMMA)); (ii) more polar systems (i.e., copoly(lactic-glycolic acid) (PLGA)) (iii) water-swelling materials (i.e., poly (hydroxyethyl methacrylate) (PHEMA)); and (iv) water-soluble materials (i.e., poly (ethylene glycol) (PEG)). On contrary, natural polymers can be broadly categorized as (i) proteins such as collagen, gelatin, and silk fibroin; (ii) polysaccharides such as chitosan, hyaluronic acid, alginate, and cellulose; (iii) and polynucleotides (DNA, RNA) [12,14]. Regardless of whether they are synthetic or natural, small repeating units constitute the long chain molecule of polymers [12,15]. In synthetic polymers, the small units (i.e., monomers) and the polymerization process influence the characteristic of the final system, such as its crystallinity, molecular weight, and melting temperature [16,17]. Some synthetic polymers lose their integrity once implanted in the body; the degradation kinetic varies from a few weeks to a few years depending on the polymers’ chemical composition and on the surrounding physiological environment [15]. Their gradual degradation begins when the surface of the system starts to absorb components such as water, proteins, and ions from the surrounding environment [15]. Of course, the degradation products released from synthetic polymers should be non-toxic for the host organism. However, as this does not always happen, the efforts of many researchers have been focused on natural polymers, which usually degrade into soluble, non-toxic chemical species [11]. These chemical species are recognizable and metabolized by the body [18], being body-friendly species. Furthermore, synthetic polymers have been partially replaced by natural ones because the latter do not lack in cells recognition sequences [19,20], and the stimulation of a chronic immunological reaction is avoided [21]. Furthermore, natural polymers show more similarity to the extracellular matrix (ECM, network of biomacromolecules including glycosaminoglycans, which are polysaccharides and fibrous proteins such as collagen, laminin, elastin, and fibronectin [12]), being readily recognizable by the body compared to the synthetic ones. Such similarity to ECM could be summarized as a suspension of macromolecules that support everything from local tissue growth to the maintenance of an entire organ. In this context, since ECM provides structural support and modulates the activity of growth factors, it represents one of the main footprints for designing biomaterials [22]. Thus, the ideal biomaterial for tissue engineering applications should be able to recreate the dynamic biochemical, structural, and mechanical properties of the naturally occurring ECM. However, despite these positive aspects, natural polymers show lower stability in terms of physical and mechanical properties compared to synthetic ones [11]. Moreover, natural polymers suffer some limitations due to their solubility and industrially acceptable processability: (i) variation in the final properties of polymer due to their source, (ii) some contamination caused by the presence of microbes, (iii) uncontrolled water uptake, and (iv) an unpredictable degradation route. Furthermore, since most natural polymers are water-soluble, various crosslinking methods to control their structure, water uptake, and degradation in aqueous environment have been developed [23,24]. Generally, natural polymers have been combined with one another to improve workability; in addition, natural polymers are typically combined with ceramics fillers (i.e., ceramics, glass-ceramics, bioactive glasses) to reinforce the structure of the final system and, thus, to produce composites with a better mechanical performance [25,26]. It is known that bioactive glasses (BGs), and especially Materials 2020, 13, 5560 3 of 38 the “gold standard” 45S5 Bioglass® [27], have been widely used in several clinical applications in regenerative medicine, tissue engineering, and dentistry [27–31] by virtue of the ability of bonding to Materials 2020, 13, x FOR PEER REVIEW 3 of 41 bone. The bonding ability to bone of BGs is mediated by the formation of bone-like hydroxy carbonate to bone.apatite The (HCA) bonding on the ability surface to bone of BGs of onceBGs is in mediated contact with by the physiological formation of fluids. bone-like The structurehydroxy and carbonatechemistry apatite of HCA (HCA) is similar on the tosurface natural of apatite,BGs once which in contact is the with mineral physiological phase of bones; fluids. thus, The thestructure deposition andof chemistry HCA on theof HCA surface is similar of biomaterials to natural is apatite, typically which considered is the mineral one of the phase initial of stepsbones; that thus, lead the to the depositionformation of HCA of a stable